The present inventor has described in U.S. Pat. Nos. 7,832,089 and 7,926,171, and in application Ser. No. 12/924,384 (each of which is incorporated herein by this reference) methods for making “microwires”, as above, extremely fine electrical wires comprising a central electrical conductor of a relatively low melting point metal in an insulative sheath of a polymer of relatively higher melting point. The wires may typically comprise a central conductor of 35 microns diameter in a sheath of 50-75 microns outside diameter. As referred to in the patents and application mentioned above, making reliable and durable electrical connections to such fine wires presents a significant technical challenge.
More specifically, the following is from cols. 18-19 of U.S. Pat. No. 7,832,089:
“Four methods of achieving a connection to the metal core of the micro-wire have been considered to date: a micro-pin system, an epoxy system, and two methods of removing the polymer sheath. The first two methods are fairly sophisticated, have not been tested, and are discussed below for completeness. Two methods of removing the polymer sheath were tested, as described below.“Depending upon the polymer sheath hardness (or brittleness), reliable connections to the microwires of the invention can potentially be achieved by a micro-pin system that punctures through the polymer coating, akin to a staple having a larger wire attached thereto, although this becomes increasingly difficult as relatively small (less than 50 microns) wires are employed. Where the metal core is less than 10 microns in diameter, the pin system must be much smaller than the core diameter of 10 microns to reduce the risk of electrical failure at the connecting point. A micro-pin system meeting these requirements has not yet been developed. Clearly, if the microwires of the invention were processed into multiconductor yarns, the odds of making good connections with one or several of the filaments using a micro-pin connector would be increased dramatically as compared with a single-filament conductor. If only signal-level currents were required to be carried, this method of making connection to the micro-wires of the invention might well be adequate.“Another method of connection that may prove satisfactory after development is to encapsulate the end of a micro-wire (or the ends of a micro-wire bundle) in an epoxy matrix and then polish the epoxy-encapsulated end to expose the micro-wires. The polished epoxy end can then be gold plated, and a connecting wire soldered thereto, establishing a connection to the core of the wire. Comparable techniques are commonly used in metallurgy when examining material under a scanning electron microscope (SEM).“A first attempt to remove the polymer sheath from the metal core utilized heat. A heated soldering iron tip was dragged across the micro-wire in an effort to deform the polymer sheath thermally. This effort was not successful. Since the polymer melts at a higher temperature than the metal, the heated tip damaged the metal core even before the polymer was partially removed. If the tip is too sharp, the tip tends to cut the metal wire while it is removing the polymer layer. In a related experiment, a heated metal bar was pushed against the micro-wire in an attempt to reach the metal core without damaging it. This was also unsuccessful. If the bar diameter was too big, the molten polymer together with the metal core was pushed away and establishing a connection to the metal core was nearly impossible.“Chemical methods of removing the polymer sheath, that is, using a chemical solvent to dissolve the polymer sheath, leaving the core untouched, proved to be more successful. The connection can then be made by soldering, possibly preceded by the epoxy-encapsulation and plating steps discussed above. A list of tested chemicals, microscopic observations, and comments are given in Table 5 of the Final Report. Of the chemicals tested, three chemicals (methylene chloride, ethylene dichloride, and N-methylpyrollidone) were ultimately used successfully to remove the outer core sheaths formed of each of Macrolon 3103, Macrolon 6457, and PETG GN007. The aggressiveness of these chemicals vary from high to low with methylene chloride being the most aggressive and N-methylpyrollidone the least. If the micro-wires were below 2 mils, the cleaning was done using the least aggressive chemical.”
From the above it will be apparent that further improvements in techniques for connecting microwires are required. In particular, as the polymer sheath provides much of the tensile strength of the microwire, connection techniques that involve the removal of the sheath are disfavored.
Further, it will be appreciated that the pin system discussed above, if implemented, could easily damage both the strength-providing polymer sheath and the current-carrying metal core. This may cause the wire to fail prematurely, especially in an application where the wire is bent repeatedly.
Still further, the epoxies that were mentioned in the quotation above were intended to be electrically-conductive epoxies, containing small conductive particles, such as tiny silver flakes or filaments, or carbon nano tubes. However, these particles are not continuous, and are embedded in a nonconductive polymer matrix. Consequently, the conductivity of connections made employing such techniques is far inferior to that of metal to metal connections. In applications where the connection must be highly conductive, these connection methods are unacceptable.
Another important concern arises from the desired use of the microwires. One primary desired use is as a fiber to be used in textile manufacturing, that is, as a component of a garment. In order that the garment will be comfortable the microwires must be flexible, which in itself is readily achieved; however, the connection means provided must be such as to not introduce any “hard points”, i.e., points or edges of connection devices or the like where the wire will tend to be flexed repetitively about a small radius as the garment is worn, where the wire is likely to fail.
More specifically, it is important that any connection method that involves a stiffening member at the end of the wire—such as an elongated metal tube into which the end of a length of microwire is inserted, so that a further electrical connection can then made to the tube—be designed so as to avoid any “hard points”, e.g., where the wire exits the tube, so that the wire does not tend to be flexed repeatedly at the end of the tube, which would tend to cause the microwire to fracture at that point.